1. Field of the Invention
This invention relates, in general, to optical printheads and, more specifically, to LED printheads for use in copiers, printers, duplicators and like devices.
2. Description of the Prior Art
Optical printheads are used in copiers, duplicators and printers to expose a photosensitive surface or film in the apparatus in such a manner that a latent image is formed on the film. The image is later developed and transferred to paper for producing a hard copy output from the apparatus. Normally, optical printheads use light emitting diodes (LED's) to generate or produce the radiation necessary to expose the photosensitive film. In conventional printheads, the LED's are arranged in a linear array of LED's having a designed density to provide a resolution of a predetermined number of dots per inch. In addition, some printheads have lens assemblies supported over the LED array to provide a modular unit which can be prefocused and quickly changed between different machines. These lenses can be a plurality of fiber optic filaments bundled together to form a linear array having dimensions approximating the dimensions of the LED array. Because of the preciseness required in high resolution printheads of both the LED array and the lens assembly, special mounting precautions must be taken to insure stress and distortion-free performance of the printhead over a wide temperature range.
One requirement in such printheads is to maintain the lens assembly as straight as possible. U.S. Pat. No. 4,821,051, issued on Apr. 11, 1989 to the same assignee as the present invention, illustrates one method by which the lens assembly can be kept straight. According to that patent, a substantially large plate surrounds the lens and keeps the lens from becoming bowed or distorted. Mounting of the printhead to the associated copier or printer is accomplished by using the surfaces on the plate as the main mounting structure. The remaining portions of the printhead are connected to the plate for support, thus the plate surrounding the lens becomes the major or most substantial component in the assembly. While this provides for an appropriate printhead in some applications, the sheer size and weight of the overall printhead can present problems in some installations.
Another requirement in printheads is to provide some mechanical stress relief between the lens and LED portions of the printhead. Stress can occur with changes in the temperature of the printhead components. According to the referenced patent, the stress relief is provided by a special slotted backing plate to which the LED's are mounted. This arrangement also provides a printhead suitable for many applications, but the extra weight, cost and complexity of the slotted backing plate may be undesirable in certain installations.
Therefore, it is desirable, and an object of this invention, to provide a modular LED printhead which properly controls the shape of the lens and the mechanical stresses without the need for large, heavy and complex structures and components in the printhead.